A polymer of a conjugated diene such as 1,3-butadiene and isoprene or a copolymer formed of a conjugated diene with a vinyl aromatic monomer (copolymerizable with a conjugated diene) such as styrene, has been widely used as an elastomer.
Such a (block) copolymer of a conjugated diene and a vinyl aromatic monomer is a thermoplastic elastomer which will not be vulcanized and used as a modifier of an impact resistant transparent resin, a polyolefin or a polystyrene resin. However, a polymer containing an olefinic unsaturated double bond has a problem in stability such as heat resistance, oxidation resistance and weather resistance due to reactivity of a double bond. Because of this, the polymer containing an olefinic unsaturated double bond is used within a limited range where the polymer is not exposed to sunlight and high temperature. Under the circumstances, in order to improve durability and oxidation resistance of the polymer, it has known that the polymer is modified by adding hydrogen to double bonds within the polymer to partially or wholly saturate them and put in use.
Various reports have been made on a general method for hydrogenating a polymer having an olefinic double bond. The hydrogenation methods are roughly divided into the following two types. The first one is a method of using a non-homogeneous catalyst such as a metal-carrying catalyst in which a noble metal such as platinum, palladium and rhodium is carried by e.g., carbon, silica or alumina. The second one is a method of using a Ziegler catalyst using nickel or cobalt, or a homogeneous catalyst such as an organic metal compound such as rhodium and titanium.
A hydrogenation reaction using a non-homogeneous catalyst has a drawback in that the hydrogenation reaction must be performed at a high temperature-high pressure condition and the catalyst being expensive must be recycled by recovering it after completion of the reaction by a filter; and has another drawback in that cost for a reaction facility is expensive. In contrast, a hydrogenation reaction using a homogeneous catalyst has advantages: since the activity of the catalyst is high, a high-yield hydrogenation reaction can be expected with a trace amount even in mild conditions such as a low temperature-low pressure condition; and facility cost is low. However, the hydrogenation reaction using a homogeneous catalyst has a drawback in that it is difficult to separate the homogeneous catalyst from a product after the reaction.
Many methods have been already known for the hydrogenation reaction using a homogeneous catalyst. Examples of the methods known in the art may include a hydrogenation method using a catalyst, which employs a compound of a metal of the VIII family of the periodic table, in particular, nickel or cobalt, in combination with an appropriate reducing agent such as an alkylaluminum compound; and a method of hydrogenating a unsaturated double bond of a conjugated diene polymer by use of a catalyst, which employs a bis(cyclopentadienyl)titanium compound in combination with an appropriate reducing agent such as an alkylaluminum compound.
The hydrogenation reaction using a homogeneous catalyst can generally realize a high hydrogenation rate and a high reproducibility, even with a small amount; however, there is still a problem in that catalyst residue in a polymer solution after the hydrogenation reaction is not easily removed. The metal component remaining in a polymer causes a reaction upon being exposed to air or UV rays, decomposes the polymer and deteriorates hue of a final polymer, reducing a commercial value. For the reasons, the metal component is desired to be removed. Generally a homogeneous catalyst is rarely separated by a physical means such as filtration performed after the reaction and must be separated by a chemical reaction.
Then, to solve this problem, several proposals have been made on a method for removing metal residue of a homogeneous catalyst remaining in a polymer solution. For example, to remove a catalyst consisting of a metal of the VIII family of the periodic table, such as nickel, Patent Literature 1 discloses a method for removing residue by treating it with an oxidizer and a dicarboxylic acid; Patent Literature 2 discloses a method for removing a catalyst by adsorbing an oxidized metal to a silicate; Patent Literature 3 discloses a method of removing a nickel catalyst reacted with oxygen by adsorbing it to active carbon; and Patent Literature 4 discloses a method of removing a nickel catalyst by use of aluminum phosphate.
So far, little has been known about a method for removing titanium residue. For example, Patent Literature 5 discloses removal of titanium residue by an inorganic acid, an alcohol and water; and Patent Literature 6 discloses removal of titanium residue by an organic acid, an alcohol and water. These are the matters just known in the art.